A. Technical Field
The present invention relates to a thermally reversible crosslinked matter and a hot melt resin comprising this crosslinked matter.
B. Background Art
A thermoplastic resin melts and thereby falls into a state of mold-processability when heated to high temperature, and then falls into a state of having strength durable to use when cooled. Therefore, the thermoplastic resin is widely and conveniently used in the world.
However, although the viscosity of the thermoplastic resin standing in a melted state due to heating, in general, fully satisfies the mold-processability, the strength or heat resistance of the thermoplastic resin is not high at temperature where the thermoplastic resin is used. Therefore, the use range of the thermoplastic resin is limited.
A block copolymer, represented by SIS (styrene-isoprene-styrene), forms a pseudocrosslinked structure due to concentration of high-Tg polymer blocks at normal temperature, and the viscosity of the block copolymer decreases at or above Tg so much that the pseudocrosslinked structure breaks and thereby falls into a state of mold-processability. Therefore, the block copolymer has the mold-processability at relatively high temperature and the strength or heat resistance at use temperature without conflict. However, because the pseudocrosslinked structure is dominated by Tg of the copolymer itself, there are problems in that the heat resistance is inherently deficient.
Many suggestions have been made that the above-mentioned problems of the thermoplastic resin should be solved by introducing into the thermoplastic resin a real, thermally reversible crosslinked structure, in other words, a crosslinked structure which breaks due to heating and then reverts due to cooling. However, any of these suggestions has problems.
JP-A-50-139135 and JP-A-51-019035 disclose thermoplastic resins in which acrylic polymers having carboxyl groups are crosslinked with metal ions. JP-A-56-014573 discloses a thermoplastic resin as produced by adding maleic anhydride to a low molecular polyisoprene and crosslinking with metal ions. In addition, JP-A-05-202345 discloses a thermoplastic resin comprising an acrylic polymer having carboxyl groups as neutralized with polyvalent metals. These thermoplastic resins containing metal-crosslinked structures are more excellent than the block copolymer, represented by SIS, with regard to the heat resistance, but have problems in that: the melt viscosity during heating is high (because the metal ion crosslinkage is so relatively strong that it is difficult to break even if heated), the thermal stability during heating is low (because, especially, in the case where an ester group is present in the polymer, the metal plays a catalytic part in forming a crosslinkage between organic high molecules, whereby a covalent bond is gradually formed between the organic high molecules), and the melting temperature gradually rises.
JP-A-58-125774 discloses a thermoplastic resin which comprises an acrylic polymer having carboxyl groups as neutralized with polyvalent metals and further comprises an o-methoxyaryl acid. This thermoplastic resin has effects in that its melt viscosity decreases during heating, when compared with the above-mentioned conventional metal ion-crosslinked matters, but has problems in that the melt viscosity of the crosslinked matter is too high when compared with a noncrosslinked one, and therefore, still in this case, the thermal stability during heating is poor because a covalent bond is gradually formed due to heating.
JP-A-52-065549 discloses a thermoplastic resin which comprises a mixture of a polymer containing acid groups with a polymer containing amino groups. JP-A-56-057865 discloses a thermoplastic resin as produced by copolymerizing (meth)acrylamide with an olefinic unsaturated mono- or dicarboxylic acid. JP-A-57-158275 discloses a thermoplastic resin which comprises a major proportion of an acrylic polymer having amino groups in the molecular inside and carboxyl groups at molecular terminals. These thermoplastic resins utilizing an ion crosslinkage as formed between the acid and the amino group also have problems in that: the formed ion crosslinkage is so strong that it is difficult to break even if heated, and the melt viscosity is therefore high; and a thermally irreversible amide bond is formed between the acid and the amino group, and therefore, still in this case, the thermal stability during heating is low.